What is the function of phospholipids in cell membranes?

Phospholipids play a critical role in the structure and function of cell membranes. They are fundamental components of the lipid bilayer, which forms the basic structure of cell membranes. The unique arrangement of phospholipids, with their hydrophilic (water-attracting) "heads" facing outward towards the aqueous environment and their hydrophobic (water-repelling) "tails" facing inward away from the water, creates a selectively permeable barrier. This barrier allows certain substances to pass through while preventing others, which is vital for maintaining the internal environment of the cell and supporting cellular homeostasis. The selective permeability is essential for processes such as nutrient uptake, waste elimination, and the maintenance of ion gradients across the membrane.

The other options do not accurately relate to the primary function of phospholipids in cell membranes. Phospholipids are not involved in regulating blood flow, providing energy storage, or facilitating DNA replication. Their main purpose is to form the barriers that dictate the movement of substances in and out of the cell.